Hydrogen has gained increasing interest in a variety of applications because it has a very high energy density per unit weight and is essentially a non-polluting agent, while the main by-product of energy release from hydrogen is water. Hydrogen can be produced from a variety of sources and processes. For instance, hydrogen can be produced via syngas generation from coal, natural gas, or hydrocarbons obtained from, e.g., fossil fuels or biomass. Beneficially, hydrogen can also be produced from more environmentally friendly techniques, such as by the electrolysis of water using nuclear, wind, or solar energy.
While hydrogen has wide potential as a fuel and in a variety of devices including, but not limited to, rechargeable batteries, pumping and compression systems, and hydrogen absorption coolers, a major drawback in its utilization has been the lack of acceptable hydrogen separation and storage mediums and systems.
Conventionally, hydrogen has been stored in the gas phase under high pressure or in the liquid phase at extremely low temperatures. Unfortunately, such storage mechanisms require expensive processing and facilities (e.g., high pressure containers and low temperature maintenance). As a result, storage of hydrogen in the solid phase as a hydride has been developed. Solid state hydrogen storage materials that have the ability to efficiently and reversibly store hydrogen are of particular interest with respect to devices that can beneficially employ a controlled absorption/desorption mechanism, particularly those having a large hydrogen-storage capacity. Reversible storage of hydrogen in a solid phase, for instance in the interstitial hydride form, can provide a greater volumetric storage density than storage as a compressed gas or a liquid. Moreover, hydrogen storage in a solid phase presents fewer safety problems than those caused by hydrogen stored in a gas or a liquid phase, particularly when desorption can be well controlled.
Solid phase storage of hydrogen in the form of an interstitial hydride commonly utilizes metals or metal alloys as the solid phase hydrogen absorbent. Interstitial hydrides are traditionally termed ‘compounds’, even though they do not strictly conform to the definition of a compound. They more closely resemble alloys such as steel, and as such are commonly described as incorporating the hydrogen via ‘metal bonding.’ In interstitial hydrides, hydrogen can exist as either an atomic or diatomic entity and the hydride is formed by the absorption and insertion of hydrogen into the crystal lattice of the metal, metal alloy, or a phase of the metal alloy. Interstitial hydride systems are usually non-stoichiometric with variable amounts of hydrogen atoms in the lattice and as such, their absorption capacity can vary greatly between materials and conditions. In general, however, metal hydride systems have the advantage of high-density hydrogen-storage that is effective for long periods of time. For instance, palladium (Pd) can absorb up to 900 times its own volume of hydrogen at room temperatures.
In addition to high storage density capability, good reversibility is desirable in solid state hydrogen storage to enable repeated absorption-desorption cycles without significant loss of hydrogen storage capabilities. Good absorption/desorption kinetics are also generally necessary to enable hydrogen to be absorbed/desorbed in a relatively short period of time. In addition to other desirable characteristics, a useful solid state hydrogen storage material and system will provide a control mechanism that can tightly control hydrogen desorption from the solid state and prevent excessive pressure build up and possible explosions.
A variety of solid state hydrogen storage materials and systems have been developed. For instance, FIG. 1 illustrates a system that includes a bed of metal hydride powder for storing hydrogen in the form of the interstitial metal hydride. In order to control absorption and desorption of the hydrogen, the system requires close temperature control. One way to accomplish this is through a hot/cold nitrogen (HCN) circulation system, as demonstrated in the illustrated system of FIG. 1, for heating and cooling the bed. Unfortunately, such a system requires the use of large compressors, heat exchangers, valves and piping that are bulky and maintenance intensive.
Accordingly, what are needed in the art are materials, systems incorporating the materials, and/or methods for using the materials that can safely and efficiently store and release hydrogen. More specifically, a system that can controllably release hydrogen from a solid state storage material in an efficient and well controlled fashion would be highly beneficial.